North American Die Casting Assoc., www.diecasting.org
Researchers at the North American Die Casting Assoc. (NADCA), Wheeling, Ill., are developing a die-casting material and process that improves mechanical properties of aluminum and magnesium alloys and lets manufacturers “dial-in” a particular coefficient of thermal expansion. The so-called HyperCast project involves composite materials containing ceramic particles in a metal matrix. What sets this method apart from traditional techniques is the way the particles get into the matrix. Rather than being mixed into the molten metal, particles are formed through an exothermic chemical reaction. HyperCasting is said to produce strong, lightweight, 100%-recyclable parts for automotive parts such as engine blocks and components, and suspension parts.
“We studied different methods of die casting, all of which are based on injecting molten metal at high pressure into closed steel dies,” says Steve Udvardy, NADCA director of research, education, and technology. “The idea for HyperCast was based on this experience.”
To understand the technology, it helps to know a little about die casting. Basically, the metal to be cast — typically zinc, aluminum, or magnesium — comes into the plant as ingots, bars, or slabs which are melted. (Some facilities buy molten alloy.) A furnace holds the molten alloy which is either pumped or ladled into the die-casting machine. A ram forces a predetermined volume of metal into the die. When the die opens, the cooled parts either fall onto a conveyor belt or are extracted by a robot or operator. Die casting is a high-production process that creates net or near-net parts to tolerances of 0.005 to 0.010 in. Dies can be reused for several thousand shots.
Conventional composite materials have particles such as silicon carbide mixed into aluminum, says NADCA Project Engineer Alex Monroe. “But it is not easy to mix in small particles. For illustration, consider trying to add cocoa powder to hot water. The powder sits on top of the water and it takes lots of vigorous stirring to get a cup of hot chocolate.”
HyperCasting, in contrast, uses self-propagating high-temperature synthesis (SHS), says Udvardy. “For example, titanium and carbon powder in a pellet is placed in the shot end of the machine or even the die itself. Molten metal is brought to a specific temperature where a chemical reaction converts titanium and carbon in the aluminum or magnesium into titanium carbide. The reaction gives off heat and continues to propagate until everything is thoroughly mixed, a process that takes mere seconds,” he says.
One project goal is to further the use of magnesium in engine blocks, says Monroe. “Magnesium is less dense than aluminum and therefore lighter. Problem is, engine blocks run at the boiling temperature of water, about 100°C. At this temperature, magnesium has poor creep resistance and may deform under low loads. Particulates in the metal can eliminate this problem. They also give the alloy better wear resistance, so there is no need to cast-in iron inserts for pistons cylinders and journal bearing areas. Injection molders can even control the percentage of particulates for application-specific coefficients of thermal expansion. The overriding objective, therefore, is a lighter engine block that meets DoE efficiency goals.” The project hopes to build such an engine in about three years.
Potentially of future interest: Producing a material that is more than 50% particulates. “This becomes a ceramic, not a metal,” says Udvardy. “Applications might include those in which you want to control a material’s electronic performance. For example, an engineer might want to build in better radioactive shielding or create specific conductive behaviors. A ceramic composite made via HyperCasting could lead to a new realm of material,” he says.